1. Field of the Invention
The present invention relates to a fast reactor having a reactivity control reflector for controlling the reactivity of a reactor core by moving reflectors upward and downward, and more particularly to a fast reactor having a reactivity control reflector which is excellent in soundness of structure and has high reliability.
This application claims priority from Japanese Patent Application No. 2007-131441, filed May 17, 2007 and Japanese Patent Application No. 2008-123952, filed May 9, 2008, which are incorporated herein by reference in their entirety.
2. Related Art
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 6-174882) discloses an example of conventional fast reactors, which is shown in FIG. 21. The conventional fast reactor 1 has a reactor core 3 which is accommodated in a reactor vessel 2 and in which a nuclear fuel assembly is loaded. The reactor core 3 is formed in an approximately columnar shape, and the outer periphery thereof is surrounded by a core barrel 4 for protecting the reactor core 3. A reflector 5 is installed outside of the core barrel 4. The reflector 5 is coupled with a reflector drive apparatus 6 through a drive shaft 7, and moved upward and downward around the reactor core 3 by driving the reflector drive apparatus 6 to thereby control the reactivity of the reactor core 3.
A cylindrical partition wall 9 is installed outside of the reflector 5 to surround the reflector 5, and a flow path of a primary coolant 8 is formed between the partition wall 9 and the reactor vessel 2. The partition wall 9 is accommodated in the reactor vessel 2, and the flow path of the primary coolant 8 and a neutron shield body 10 are installed. The neutron shield body 10 is installed so as to surround the reactor core 3.
The reactor core 3, the core barrel 4, the partition wall 9, and the neutron shield body 10 are all mounted on a reactor core support plate 11 so as to be supported thereby. An electromagnetic pump 12 is installed above the neutron shield body 10 to circulate the primary coolant 8, and an intermediate heat exchanger 13 is installed above the electromagnetic pump 12. The intermediate heat exchanger 13 performs heat exchange of the primary coolant 8 and a secondary coolant and heats the secondary coolant. The secondary coolant flows from an inlet nozzle 14 into the intermediate heat exchanger 13. After the secondary coolant is subjected to the heat exchange by the intermediate heat exchanger 13 and heated, it is supplied to a steam generator, not shown, from an outlet nozzle 15.
Further, the reflector 5 located around the reactor core 3 of the reactor vessel is arranged as shown in FIG. 22 (refer to Patent Document 2: Japanese Patent Application Laid-Open Publication No. 6-51082). The reflector 5 for controlling the reactivity of the reactor core 3 is composed of a lower neutron reflecting portion 5a and an upper cavity portion 5b. The cavity portion 5b is installed on the neutron reflecting portion 5a and formed of a box member in which a vacuum or a gas 17, which have a neutron reflection capability inferior to that of the coolant 8, is enclosed. The cavity portion 5b can suppress a core reactivity lower than a state in which the outside of the core barrel 4 is covered with the primary coolant 8. It is intended to increase the enrichment of a nuclear fuel by reducing the core reactivity to thereby increase the reactivity life of the reactor core 3.
In conventional fast reactors having the reactivity control reflector, the temperature of the primary coolant 8 is 300° to 550°, about 500° on the reactor core 3 side in the core barrel 4, and about 350° on the neutron shield body 10 side of the partition wall 9, and thus, a temperature difference of about 150° is set between the core barrel 4 and the partition wall 9.
Further, when the primary coolant 8 is reversed on the bottom of the reactor vessel 2, moved upward, and passes through the reactor core 3, since it is heated from about 350° to 500°, the coolant temperature in the core barrel 4 has a temperature difference of about 150° in an axial direction.
Accordingly, since a temperature difference is generated to the neutron reflecting portion 5a and the cavity portion 5b of the reflector 5 in the radius direction and the axial direction thereof, the reflector 5 is thermally deformed by thermal expansion difference due to the temperature difference. When the reactor is shutdown in an emergency due to the deformation of the reflector 5 and the reflector 5 is dropped, there is considered a possibility that the reflector 5 cannot be dropped within a predetermined drop time because it comes into contact with the core barrel 4 and the partition wall 9 in the space therebetween.
In addition to the above, it is also considered that the reflector 5 may be damaged by thermal stress and creep caused by the temperature difference in the reflector 5. Further, when a box-shaped cavity portion is employed as the cavity portion 5b of the reactor 5, a problem arises in how the cavity portion 5b of the reflector 5 is to be arranged to prevent damage and breakage of the box member to prevent buckling due to thermal expansion difference caused by the temperature difference between the core barrel 4 and the partition wall 9.